1. Field of the Invention
The present invention relates to a printing system and a printing method, and more specifically, to a printing system composed of a plurality of printing apparatuses connected together so as to communicate with each other and computer apparatuses, as well as a printing method.
2. Description of the Related Art
Printing apparatuses constituting a conventional printing system, particularly those which carry out color printing commonly incorporate a function to control printing so as to achieve constant color reproduction for printed matter. This function is generally called a calibration process. An object of the calibration process is to control printing so that three-color or four-color print ink or toner used for printing exhibits a constant gradation characteristic.
Now, description will be given of the reason why the calibration process is required for a printing apparatus to which what is called an electrophotographic process is applied, for example, a laser printer (hereinafter referred to as an LBP).
Electrophotography commonly uses a print processing system that basically executes a procedure to irradiate a photosensitive member with laser light to form an electrostatic latent image allowing charged toner (powdery ink) to adhere to the electrostatic latent image for development, transferring this image to a sheet, and using heat and pressure to fix the image to the sheet. Color LBPs or copiers use a method of using three-color toner (yellow, magenta, and cyan; these colors are often represented as YMC) or four-color toner (yellow, magenta, cyan, and black; these colors are often represented as YMCK or YMCB. The present invention uses the YMCK expression. In the following description, the color LBP is assumed to execute printing in four-colors including Y, M, C, and K. However, the present invention is obviously effective on color LBPs that execute color printing in three-colors) to draw an image, forming this image on a surface of an intermediate transfer member, and transferring the image to a sheet for fixation.
In this electrophotographic process, the factors of variations in gradation characteristic include environmental temperature and humidity. Variations in environmental temperature or humidity affect the electrophotographic process as described below.
1) The environmental temperature or humidity may change the state of an electrostatic latent image formed on the photosensitive member to change the density characteristic of an image provided in the next development step.
2) The environmental temperature or humidity may change the charged state of toner to change the density characteristic of an image provided in the next development step.
3) The environmental temperature or humidity may change the rate of transfer from a photosensitive drum to the intermediate transfer member to change the density characteristic of an image formed on the intermediate transfer member.
4) The environmental temperature or humidity may change the rate of transfer from the intermediate transfer member to a sheet to change the density characteristic of an image formed on the sheet.
5) The environmental temperature may change a fixation temperature and thus the dissolution level of toner during fixation to change coloring.
As described in 1) to 5), a variation in environmental temperature or humidity may vary the density characteristic in a plurality of steps of the electrophotographic process. That is, the calibration process can be considered to achieve constant color expression in spite of a variation in environmental temperature or humidity.
Now, a common calibration process will be described in further detail.
The LBP has the intermediate transfer member that forms a color image on a sheet which image is to be transferred. In the calibration process, a plurality of fine areas with a predetermined density, such as those called patch patterns, are formed on the intermediate transfer body. Then, a density sensor is used to read the actual densities of the individual patch patterns to determine actual characteristics corresponding to the relationship between specified densities and the actual densities. The patch patterns formed on the intermediate transfer member are deleted after they have been read by the density sensor.
FIG. 3 is a diagram showing an example of a patch pattern formed on the intermediate transfer member. In this figure, reference numeral 301 denotes the entire patch pattern, and reference numerals 311, 312, 313, and 314 denote rectangular areas (patches) of density 20%, 40%, 60%, and 80%, respectively. These densities are, for example, numerical values in a density expression using dither which values correspond to pixels on which toner is placed.
FIG. 4 is a view schematically showing how patch patterns formed on the intermediate transfer member are read. In the figure, reference numeral 401 denotes the intermediate transfer member. The patch pattern 301 is formed on the intermediate transfer member 401. A density sensor 402 reads the different densities of the patches. A printer controller 403 is connected to the density sensor 402.
When the patch pattern 301 is thus formed and read, it exhibits a characteristic such as the one shown in FIG. 5A. In FIG. 5A, the four points shown as black circles indicate densities read from the 20%, 40%, 60%, and 80% patches, respectively, using the density sensor 402. The whole characteristic is calculated from these four points. The calculated characteristic curve is shown as a solid line in FIG. 5A. On the other hand, the linear characteristic shown by a broken line in FIG. 5A is assumed to be realized by corrections (this characteristic will hereinafter referred to as an ideal characteristic). Then, a corrected characteristic such as the one shown by a solid line in FIG. 5B can be determined.
Specifically, input density levels having density values corresponding to the ideal characteristic shown by the broken line in FIG. 5A are determined from the density characteristic curve shown as the solid line in FIG. 5A. These levels are represented as a characteristic curve such as the one shown in FIG. 5B, which curve indicates a density converted characteristic.
When the input densities are utilized after being converted in accordance with the density converted characteristic shown in FIG. 5B, output densities corresponding to the input density levels exhibit an ideal characteristic as shown in FIG. 5C. Furthermore, it is assumed that a color is to be expressed in YMCK. Then, once the relevant density levels are determined, the target color can be expressed using the densities converted in accordance with the density converted characteristic shown in FIG. 5B.
In the above description, the patch pattern with the four densities, 20%, 40%, 60%, and 80%, are used. However, the densities may be set in smaller increments. However, setting the densities in smaller increments increases the amount of information to be processed and thus the number of patch readings. This increases the time required for the whole calibration process.
Now, timing for calibration will be described.
As described above, the gradation characteristic of electrophotography tends to vary with the environmental temperature or humidity. On the other hand, the calibration process is a function to stably reproduce the colors in spite of a variation in environmental conditions. Accordingly, if the environmental temperature or humidity varies, a series of calibration operations must be performed over again, including the formation and reading of the patch pattern 301, the calculation of an actual density characteristic (the characteristic shown by the solid line in FIG. 5A), and the creation of a density converted characteristic (the characteristic shown by the solid line in FIG. 5B). In general, the calibration is carried out if the internal temperature of the printing apparatus varies upon power-on or owing to continuous printing. The calibration is also carried out when the number of sheets printed is counted and reaches a specified value or depending on the time elapsing after power-on.
As described above, the calibration process is essential in stably reproducing the colors. However, the calibration process uses a method of forming patches on the intermediate transfer member and measuring the densities of the patches using the density sensor. Thus disadvantageously, it is impossible to execute a printing process, the original purpose of the LBP, during the calibration process.